Traditional marine pilings are made of steel, concrete, or wood. Steel and concrete are very heavy and expensive and do not have desired resiliency for fendering applications. Steel is especially subject to rapid corrosion in a marine environment. Wood suffers from rapid erosion and is subject to attack by marine animals which deplete its effectiveness. In order to prolong its useful life, wood used, e.g., for marine pilings, telephone poles, and railroad ties, is typically treated with a preservative, such as creosote. However, creosote and other preservatives are detrimental to the environment. Furthermore, given the recent efforts for preservation of forests, the use of wood pilings, poles and ties is not desirable.
Marine pilings made of plastic have been proposed. For example, U.S. Pat. No. 5,051,285 discloses a structural plastic member suitable for use as a plastic piling. A steel pipe is positioned in a mold and coated with thermoplastic resins, fillers, and additives. The plastic is cooled and the resultant plastic member is then removed from the mold.
This approach suffers many disadvantages. Marine pilings typically vary in length from ten to eighty feet and have a diameter as small as three inches depending on a specific application. As a piling manufacturer must either construct molds of varying sizes, which is very expensive, or use a single mold to produce pilings of a certain length and diameter and join multiple pilings longitudinally to achieve the desired length.
The use of a mold also limits the length of a piling which can be produced. The plastic in the mold must be in a flowable state throughout the entire process of filling the mold. The flowable state becomes difficult to maintain as the length and size of the structure is increased. Additionally, the adhesion of the plastic to the pipe is difficult to control in such an operation where the plastic melt is introduced at one end of an elongated mold and required to stick to the metal core pipe at the opposite end, which is typically at least ten feet away. It is believed that such a formed structure would contain hollows or at least weak areas formed by interfaces between melt streams of different relative ages.
Because the length of the member is limited by mold size, the structure disclosed in U.S. Pat. No. 5,051,285 must be connected to other such structures to form pilings of the length required for a given application. Such joining methods and means are expensive, cumbersome and leave potential seams for water and other environmental factors to attack the metal pipe core. Regardless of the production method, plastic pilings must be properly cooled so that the plastic maintains its appropriate shape. Such a cooling process may be lengthy, particularly since pilings typically exceed 10 inches in diameter and 30 feet in length. If cooling time is sacrificed, the piling may bend or sag from its desired shape.
The foregoing problems with respect to marine pilings have been solved to a great extent by the methods and apparatus disclosed in related U.S. Pat. Nos. 5,650,224 and 5,658,519, both of which are assigned to the assignee of the present invention and the subject matter of which is incorporated herein by reference. In the methods disclosed in the aforesaid applications, the continuously extruded members are reinforced by a plurality of substantially rigid rods or rebar that are fed to the extrusion die continuously or in discrete lengths.
The use of steel or glass fiber reinforcing rods such as steel rebar or pultruded glass fiber rebar of given lengths in a continuous extrusion process requires that the individual rods or rebar be connected together end to end. Because of the high tensile forces acting on the reinforcing rods during the extrusion process, the joint or splice between the reinforcing rods, particularly the glass fiber rebar, must be extremely reliable. Failure of such a splice can result in scrapping of an entire extruded piling of thirty to one hundred feet in length having a diameter of eight to sixteen inches or more.
It has been found that all the joints between the reinforcing rods must be located at the free ends of the extruded member rather than intermediate the length of the member. Otherwise, the strength of the piling will be adversely affected. One difficulty in forming continuously extruded reinforced plastic members of such large diameter as marine pilings, telephone poles and the like, is that when the member is cut, the possibility exists that the plastic has not completely solidified across the entire cross-section thereof. This occurs even when a central plastic core is used as a heat sink to enhance cooling and solidification of the molten plastic. Accordingly, it is possible during the cutting of the extruded member at the reinforcing rod joints that molten plastic will leak from the cut end of the member resulting in internal voids in the extruded member.
Therefore, there is also a need for a reliable joint connection, especially between the ends of pultruded glass fiber rebar, as well as a method for determining the location of the reinforcing rod joints in the extruded member so that the member can be cut precisely at the joints thereby providing an extruded member with joint-free reinforcing rods. Furthermore, there is a need for a way to prevent leakage of molten plastic from the extruded member when it is cut at the reinforcing rod joints.